Part of the armature is not copied and pasted correctly
So, I would NEVER use Copy and Paste for a Mesh and Armature combination. I would always duplicate in the same Blender project by selecting BOTH in OBJECT mode and then key SHIFT+D and move the copies.

To move to another Blender project, I would ALWAYS append the mesh and armature in one operation from the first blend file to the second. To append, click Append in the file menu and select mesh object, if you have parented them properly, the armature will come with it, in its entirety.

At around the 1 minute mark in your video there is an error message at the top, look in the console and tell me what that was…

Clockmender.